It has become increasingly common to treat a variety of medical conditions by introducing a medical device into the vascular system or other lumen within a human or veterinary patient such as the esophagus, trachea, colon, biliary tract, or urinary tract. For example, medical devices used for the treatment of vascular disease include stents, catheters, balloon catheters, guide wires, cannulas and the like. While these medical devices initially appear successful, the benefits are often compromised by the occurrence of complications, such as late thrombosis, or recurrence of disease, such as stenosis (restenosis), after such treatment.
Restenosis, for example, involves a physiological response to the vascular injury caused by angioplasty. Over time, de-endotheliaziation and injury to smooth muscle cells results in thrombus deposition, leukocyte and macrophage infiltration, smooth muscle cell proliferation/migration, fibrosis and extracellular matrix deposition. Inflammation plays a pivotal role linking early vascular injury to the eventual consequence of neointimal growth and lumen compromise. In balloon-injured arteries, leukocyte recruitment is confined to early neutrophil infiltration, while in stented arteries, early neutrophil recruitment is followed by prolonged macrophage accumulation. The widespread use of coronary stents has altered the vascular response to injury by causing a more intense and prolonged inflammatory state, due to chronic irritation from the implanted foreign body, and in the case of drug eluting stents (DES), from insufficient biocompatibility of the polymer coating.
Over the past several years, numerous local drug delivery systems have been developed for the treatment and/or the prevention of restenosis after balloon angioplasty or stenting. Examples include local drug delivery catheters, delivery balloon catheters, and polymeric drug coated stents. Given that many diseases affect a specific local site or organ within the body, it is advantageous to preferentially treat only the affected area. This avoids high systemic drug levels, which may result in adverse side effects, and concentrates therapeutic agents in the local area where they are needed. By treating just the diseased tissue, the total quantity of drug used may be significantly reduced. Moreover, local drug delivery may allow for the use of certain effective therapeutic agents, which have previously been considered too toxic or non-specific to use systemically.
One example of a local delivery system is a drug eluting stent (DES). The stent is coated with a polymer into which drug is impregnated. When the stent is inserted into a blood vessel, the polymer degrades and the drug is slowly released. The slow release of the drug, which takes place over a period of weeks to months, has been reported as one of the main advantages of using DES. However, while slow release may be advantageous in the case where a foreign body, such as a stent, is deployed, which is a source of chronic irritation and inflammation, if a foreign body is not implanted it is instead advantageous to rapidly deliver drug to the vascular tissue at the time of treatment to inhibit inflammation and cellular proliferation following acute injury. Thus, a considerable disadvantage of a DES, or any other implanted medical device designed for sustained release of a drug, is that the drug is incapable of being rapidly released into the vessel.
Additionally, while drug-eluting stents were initially shown to be an effective technique for reducing and preventing restenosis, recently their efficacy and safety have been questioned. A life-threatening complication of the technology, late thrombosis, has emerged as a major concern. Drug eluting stents cause substantial impairment of arterial healing, characterized by a lack of complete re-endothelialization and a persistence of fibrin when compared to bare metal stents (BMS), which is understood to be the underlying the cause of late DES thrombosis. Concerns have also been raised that the polymeric matrix on the stent in which the anti-proliferative drug is embedded might exacerbate inflammation and thrombosis, since the polymers used are not sufficiently biocompatible. These polymeric systems are designed to facilitate long-term sustained release of drug over a period of days, months, or years, not over a period of seconds or minutes. Rapid release of drug, an intent of embodiments of the present invention, from these polymeric systems is not possible. Thus, combining a therapeutic agent with a polymer in a medical device coating may have significant disadvantages.
Another important limitation of the DES is that the water insoluble drugs are not evenly distributed in the polymeric matrix of the coating. Furthermore, drug and polymer are concentrated on the struts of the stent, but not in gaps between the struts. The non-uniform distribution of drug causes non-uniform drug release to the tissue of the vessel walls. This may cause tissue damage and thrombosis in areas exposed to excess drug and hyperplasia and restenosis areas that are undertreated. Thus, there is a need to improve the uniformity of drug delivery to target tissues by improving drug solubility in coatings of medical devices by increasing the drug's compatibility with carriers in the coatings, such as a polymeric matrix, thereby eliminating or reducing the size of drug crystal particles in the polymeric matrix or other coating to create a uniform drug distribution in the drug coating on the medical device.
Yet another important limitation of the DES is that only a limited amount of an active agent can be loaded into the relatively small surface area of the stent.
Non-stent based local delivery systems, such as balloon catheters, have also been effective in the treatment and prevention of restenosis. The balloon is coated with an active agent, and when the blood vessel is dilated, the balloon is pressed against the vessel wall to deliver the active agent. Thus, when balloon catheters are used, it is advantageous for the drug in the coating to be rapidly released and absorbed by blood vessel tissues. Any component of the coating that inhibits rapid release, such as a lipid or polymer or an encapsulating particle, is necessarily disadvantageous to the intended use of the balloon catheter, which is inflated for a very brief period of time and then removed from the body.
Hydrophilic drugs, such as heparin, have been reported to be deliverable by polymeric hydrogel coated balloon catheters. However, a polymeric hydrogel coating can not effectively deliver water insoluble drugs (such as paclitaxel and rapamycin), because they can not mix with the hydrogel coating.
The iodine contrast agent iopromide has been used with paclitaxel to coat balloon catheters and has some success in treatment of restenosis. It was reported that contrast agent improves adhesion of paclitaxel to the balloon surface. Iodinated contrast agents suffer from several well known disadvantages. When used for diagnostic procedures, they may have complication rates of 5-30%. These agents are associated with the risk of bradycardia, ventricular arrthymia, hypotension, heart block, sinus arrest, sinus tachycardia, and fibrillation. Iodine contrast agents may also induce renal failure, and as a result there are significant efforts to remove these contrast agents from the vascular system after diagnostic procedures.
Iodinated X-ray contrast agents are large hydrophilic spherical molecules. They are characterized by an extracellular distribution and rapid glomerular filtration and renal excretion. They are unable to cross membrane lipid bilayers to enter cells of the vasculature because they are large, polar, hydrophilic molecules. They are therefore not optimally effective at carrying hydrophobic drugs such as paclitaxel into cells, and the percent of paclitaxel reported to be taken up by vascular tissue after deployment of these devices is only 5-20%.
Alternatively, balloon catheters are reported to have been coated with hydrophobic therapeutic agents that have been mixed with oils or lipids or encapsulated in particles such as liposomes or polymers. All of these drug delivery formulations have significant disadvantages. Unlike hydrophilic contrast agents, oils and lipids mix well with water-insoluble drugs such as paclitaxel or rapamycin, but the particle sizes of oils used for solubilizing the therapeutic agents are relatively unstable, ranging in a broad particle size distribution from several hundred nanometers to several microns in diameter.
Loading capacity of conventional micelles is low. Another disadvantage of oil-based liposome formulations is the dependence of drug absorption on the rate and extent of lipolysis. Lipolysis of oil-based triglycerides is difficult and dependent upon many factors, and triglycerides must be digested and drug released in order to be absorbed by diseased tissue. The amount of hydrophobic drug delivered to tissues by these agents will be low, because liposomes and micelles cannot efficiently release hydrophobic drug, which they carry away before it can be absorbed by tissues. Oils and lipids are therefore not effective at rapidly and efficiently facilitating tissue uptake of drug during a very brief device deployment time, and no report has shown these types of coatings to be effective.
Drug that is encapsulated in polymeric particles may take even longer to diffuse from the coating and will have further difficulty permeating target tissues rapidly. Microspheres formed with polymeric materials, such as polyesters, when used to encapsulate water insoluble drugs, are unable to release the drug until the polymeric material is degraded. Thus, these polymeric microspheres are useful for sustained release of drug over a long period of time but cannot rapidly release drug and facilitate tissue uptake.
Combining drugs and medical devices is a complicated area of technology. It involves the usual formulation challenges, such as those of oral or injectable pharmaceuticals, together with the added challenge of maintaining drug adherence to the medical device until it reaches the target site and subsequently delivering the drug to the target tissues with the desired release and absorption kinetics. Drug coatings of medical devices must also have properties such that they do not crack upon expansion and contraction of the device, for example, of a balloon catheter or a stent. Furthermore, coatings must not impair functional performance such as burst pressure and compliance of balloons or the radial strength of self- or balloon-expanded stents. The coating thickness must also be kept to a minimum, since a thick coating would increase the medical device's profile and lead to poor trackability and deliverability. These coatings generally contain almost no liquid chemicals, which typically are often used to stabilize drugs. Thus, formulations that are effective with pills or injectables might not work at all with coatings of medical device. If the drug releases from the device too easily, it may be lost during device delivery before it can be deployed at the target site. If the drug adheres too strongly, the device may be withdrawn before the drug can be released and absorbed by tissues at the target tissues.
Thus, there is still a need to develop highly specialized coatings for medical devices that can rapidly deliver therapeutic agents, drugs, or bioactive materials directly into a localized tissue area during or following a medical procedure, so as to treat or prevent vascular and nonvascular diseases such as restenosis. The device should quickly release the therapeutic agent in an effective and efficient manner at the desired target location, where the therapeutic agent should rapidly permeate the target tissue.